Hydraulic systems, such as those found in excavators and the like, employ a hydraulic cylinder to raise and lower relatively heavy loads and at times to support such loads in an elevated position. When the cylinder is required to support the load in such an elevated position, it is normally desirable to isolate the relatively high load generated pressure in the load supporting end of the cylinder from the remainder of the system. This is to prevent the downward drifting of the load due to leakage past a valve spool of a conventional control valve normally used in such systems. The load pressure is also normally isolated to prevent the sudden dropping of the load in the event of a hydraulic line failure or the like.
This isolation can be accomplished by the positioning of a load check valve in the hydraulic line leading from the control valve to the hydraulic cylinder. Such a load check valve permits the free flow of fluid to the cylinder, but normally prevents the escape of fluid from the cylinder. The load check valve can be of the type which is vented behind the check valve spool, such that the check valve closes the hydraulic line when the venting line is blocked. When the venting line is opened, hydraulic fluid can flow from the cylinder to the control valve and from the control valve to the cylinder. The load check valve can be mounted directly to the hydraulic cylinder eliminating the need for a conduit to connect the cylinder to the check valve and thus eliminating the possibility of a break therein.
However, there is always a possibility that a rupture might occur in the hydraulic line which connects the load check valve to the control valve. If such a rupture occurs and if the venting line from the load check valve is not closed, the load can fall until the operator of the hydraulic system realizes that the load is falling and acts to block the vent line, preventing the load from dropping still further.